The present invention relates to a method for producing N, N'-bis-salicyloyl hydrazine by catalytic reaction of salicylic acid alkyl esters with hydrazine or salicylic-acid hydrazide.
In the presence of copper, many organic materials, particularly polymers such as polyolefins, but also other polymers such as polyoxymethylene, polyamide and unsaturated polyester resins which currently are in use in electrical engineering for insulating purposes, are subject to accelerated thermo-oxidative aging, which degrades their electrical and mechanical long-term use properties considerably. Especially serious is the damaging effect of copper at elevated temperatures, since the rate of aging of the polymers increases sharply with increasing temperatures.
Cross-linked polyolefins, which are used increasingly as insulating material for cables and wires, particularly are subject to strongly accelerated aging in the presence of copper and must, therefore, be protected effectively against the oxidation-accelerating influence of copper. According to experience, this can be accomplished by inserting a foil as a separating layer between the copper conductor and the insulation, or by using tinned conductors, thereby avoiding direct contact between the copper and the insulation. Such measures, however, are expensive and very cumbersome in production. Thus, for example, if a foil is used as the separating layer, only low production rates are possible, particularly for small conductor cross sections.
Another simple processing approach for obtaining the desired requirements as to quality and thermal stability of polymer materials in contact with copper involves the use of so-called copper deactivators, which inhibit the oxidation-accelerating effect of copper even at elevated temperatures. Stabilizing polymer materials by copper deactivators is, therefore, a cost-effective measure.
It is known from U.S. Pat. No. 3,849,492 to use copper deactivators of the N, N'-bis-salicyloyl hydrazine type. Multiple alkyl or alkoxy-substituted derivatives of the base compound allegedly have been found to be particularly effective for stabilizing polyolefins against the damaging effect of copper and other transition metals.
From German Pat. No. 27 03 558 (column 2, lines 49 to 65) and the corresponding South-African Pat. No. 78/0086 it is known that for a permanent stabilization of polymers in contact with copper, a combination of N, N'-bis-salicyloyl-hydrazine as the metal deactivator and oligomeric 2,2,4-trimethyl-1,2-dihydroquinoline as an oxidation inhibitor is advantageous. This stabilizing combination has been found to be particularly effective also for stabilizing cross-linked polyolefins. The high effectiveness of N, N'-bis-salicyloyl hydrazine as a metal deactivator has been determined in extensive investigations on model conductors and by aging tests on commercial products.
In the mentioned stabilizer combination, the metal deactivator employed was a commercial product which was obtained directly by reaction of salicylic-acid hydrazide with salicylic acid, adding thionyl chloride and pyridine in chlorobenzene and, after purification by washing with alcohol, was present in a degree of purity of 99% see British Pat. No. 1,398,360).
Mixing a metal deactivator of the type mentioned (as well as further additives) into a polymer material such as, for example, a polyolefin for cable and wire insulation, is accomplished on a technical scale by conventional mixing processes. In cable and wire technology, so-called concentrates consisting of a metal deactivator or other additives and polymer material, preferably are prepared first and then are processed by mixing them into further polymer material to form insulating mixtures with the desired concentrations of metal deactivator and the further additives. In the preparation of the concentrates with the metal deactivator prepared in this manner on a commercial scale, it has been found that eye irritations or eye damage occurs in persons engaged therein.
To determine the cause of eye irritations or eye damage, tests with rabbits were made. Since it must be assumed that the copper deactivator can get into the eye in solid form, for example, as dust during the technical processing, the solid substance was placed, repeatedly, directly into the eye of the test animals at relatively short time intervals, for example, daily, and the changes in the eye were observed over an extended period of time. As a result of these tests, it was found that the eye irritations and eye damage are due to the N, N'-bis-salicyloyl hydrazine itself or to small quantities of impurities due to the manufacturing process.
From U.S. patent application Ser. No. 173,409, filed July 29, 1980 and now U.S. Pat. No. 4,446,260, it can be seen that eye irritations or eye damage can be considerably reduced if an N, N'-bis-salicyloyl-hydrazine is used which is obtained by reaction of salicylic-acid alkyl esters with hydrazine or salicylic acid hydrazide. This reaction can occur in the presence of nucleophilic and/or electrophilic catalysts, nucleophilic catalysis being preferred. Particular catalysts include primary, secondary or tertiary amines as well as amino group-containing compounds such as nitrogen-containing heterocycles, amides and hydrazines, or the ammonium salts of these amines and amino group-containing compounds with inorganic or organic acids.
A disadvantage of the reaction utilizing nucleophilic catalysts of the type mentioned above, however, is that the product is produced in a form which is colored yellowish by additives, therefore requiring further processing, such as by recrystallization, for purification. On the other hand, attempting to conduct the reaction in the absence of catalysts, or with protonated acids as catalysts, results in very low yields of product.